e
seventeenth magnitude is so very small, that it may be neglected in such
an estimation. He finds, indeed, that if there are stars so low as the
twentieth magnitude, one hundred millions of them would only be equal in
brightness to a single first-magnitude star like Vega. On the other
hand, it is possible that the light of the sky at night is not entirely
due to starlight, but that some of it may be caused by phosphorescent
glow.

CHAPTER XXVI

THE STELLAR UNIVERSE--_continued_

It is very interesting to consider the proper motions of stars with
reference to such an isolated stellar system as has been pictured in the
previous chapter. These proper motions are so minute as a rule, that we
are quite unable to determine whether the stars which show them are
moving along in straight lines, or in orbits of immense extent. It
would, in fact, take thousands of years of careful observation to
determine whether the paths in question showed any degree of curving. In
the case of the more distant stars, the accurate observations which have
been conducted during the last hundred years have not so far revealed
any proper motions with regard to them; but one cannot escape the
conclusion that these stars move as the others do.

If space outside our stellar system is infinite in extent, and if all
the stars within that system are moving unchecked in every conceivable
direction, the result must happen that after immense ages these stars
will have drawn apart to such a distance from each other, that the
system will have entirely disintegrated, and will cease to exist as a
connected whole. Eventually, indeed, as Professor Newcomb points out,
the stars will have separated so far from each other that each will be
left by itself in the midst of a black and starless sky. If, however, a
certain proportion of stars have a speed sufficiently slow, they will
tend under mutual attraction to be brought to rest by collisions, or
forced to move in orbits around each other. But those stars which move
at excessive speeds, such, for instance, as 1830 Groombridge, or the
star in the southern constellation of Pictor, seem utterly incapable of
being held back in their courses by even the entire gravitative force of
our stellar system acting as a whole. These stars must, therefore, move
eventually right through the system and pass out again into the empty
spaces beyond. Add to this; certain investigations, made into the speed
of 1830 Groombridg